The invention refers to a non-capsulated housingless pressure sensor for measuring the pressure in a liquid or gaseous medium and, in particular, to a pressure sensor with a resistance unit of strip conductors arranged on a substrate in the form of a semiconductor chip.
In measuring liquid pressure, e.g. blood pressure within a blood vessel, it is known to use a pressure sensor within a catheter, which comprises a semiconductor chip having a resistance unit provided thereon. Below the resistance unit, the semiconductor chip forming the substrate is provided with a recess so that a very thin wall thickness is given at the measuring site. Due to the action of the pressure, the thin wall is bent, whereby different degrees of extension occur in the resistors of the resistance unit. Thus, it is possible to determine the pressure acting on the substrate from the voltage change measured at the resistance unit. The substrate is adhered to a support wall which may in turn be situated in a catheter, the resistance unit being located on the side of the substrate averted from the support wall. Since this side is exposed to the medium whose pressure is to be measured, it is coated with an additional passivation layer that prevents contact between the medium and the resistance unit or the substrate, respectively. It is true that all electric components of the pressure sensor are covered, but electrocorrosion can occur, in particular when the pressure sensor is operated in electrically conductive liquids, which electrocorrosion may cause a loss of the sensor or an inherent sensor drift. Moreover, possible damages of the passivation layer have a direct effect on the functional behavior of the sensor. Finally, the use of such pressure sensors in the blood circulation poses certain problems, since fluid-related erroneous measurements and deposition of blood components may occur.
From DE 39 37 522 A1, a semiconductor pressure sensor is known that has a support wall and a semiconductor substrate. A pressure transmission opening is provided in the substrate. The opening is formed as a recess in the substrate and delimited by a membrane. The resistance unit is located on the side of the substrate that abuts on the support wall. Between the substrate and the support wall, an elastomer seal is provided. However, the latter does not extend over the membrane region.
It is the object of the present invention to provide a non-capsulated housingless pressure sensor that is insensitive to damage and which can, in particular, be used as a small-sized high precision sensor.
According to the present invention, the object is solved with the features of claim 1.
In the present pressure sensor, the side of the substrate on which the resistance unit is provided faces the support wall, with an elastic intermediate layer being provided between the support wall and the substrate. With this pressure sensor, the electric components provided on the substrate are protected between the substrate and the support wall. Should external influences cause damages to the sensor, these remain non-critical for the function because only the top surface of the substrate or the chip is affected. Because of the protected accommodation of the resistance unit, the sensor can be operated in electrically conductive liquids, without electrocorrosion causing sensor loss or an inherent sensor drift. In as far as insulating layers are required, these can be kept extremely thin. The present housingless pressure sensor design results in a small-sized and flat arrangement and can be integrated in a thin support wall of a few tenths of a millimeter without protruding. The support wall may be planar or bent at a tube. Further, the pressure sensor is insensitive to light, since the electrically and possibly photogalvanically active surface is arranged in a light-tight manner between the substrate and the support wall. The present invention provides for an extremely flat sensor means without any additional sensor housing, the sensor means even being adapted to measure pressures in electrically conductive media. The insulating intermediate layer between the support wall and the substrate is elastic. This means that this intermediate layer allows for certain movements of the sensor relative to the support wall so that the substrate is held on the support wall in a floating manner, so to speak. Thereby, the sensor is mechanically decoupled from the support wall and it is made sure that mechanical or temperature-related deformations of the support wall cause no substantial electrical signals. Preferably, silicone material is used for the intermediate layer.
According to a preferred embodiment of the invention, the resistance unit is provided on a thin membrane of the substrate which is flush with the substrate surface facing the support wall, a recess being provided behind the membrane. The diameter of the recess is in the order of 0.1 to 0.6 mm so that the recess is too small to reach and damage the sunk-in thin membrane with usual tools such as pincers.
Another advantage is that wires connected to the bottom surface of the substrate can be glued or cemented directly to the support wall, whereby stress relief is obtained at the connection site. Further, the connection area whose insulation is critical, is mechanically protected due to its being arranged between the substrate and the support wall.
The present pressure sensor may be designed as an absolute pressure sensor. Here, the recess in the substrate is evacuated and covered with a vacuum-tight layer. On the other hand, it is possible to design the pressure sensor as a differential pressure sensor, where both sides of the support wall, and thus also both sides of the thin membrane, are subjected to different pressures.